The ongoing surge in the development of lightweight and space economical electronic devices has raised the need for lighter and smaller batteries as their power supplies. To meet this requirement, sealed nonaqueous electrolyte secondary batteries represented by lithium-ion secondary batteries, which are compact, lightweight, high capacity, chargeable and dischargeable, have been in practical use for compact camcorders, mobile phones, notebook computers, and other mobile electronics and communications equipment, for example.
Referring to FIGS. 4 and 5, a typical structure of such sealed nonaqueous electrolyte secondary batteries will be described. FIG. 4 is a perspective view showing a longitudinal section of a cylindrical nonaqueous electrolyte secondary battery disclosed in JP-2001-15155-A. FIG. 5 is an enlarged partial cutaway view of the sealing body shown in FIG. 4. This nonaqueous electrolyte secondary battery 10 is manufactured by the following process: rolling a positive electrode plate 11 and a negative electrode plate 12 with a separator 13 therebetween to provide a spiral electrode unit 14, placing insulators 15 and 16 on the top and bottom of the electrode unit 14 and thereafter laying the electrode unit in a steel cylindrical outer can 17 also serving as a negative electrode terminal, welding a current collecting tab 12a of the negative electrode plate 12 to the inside bottom of the outer can 17 and welding a current collecting tab 11a of the positive electrode plate 11 to a bottom plate of a sealing body 18D with a built-in safety device, injecting a predetermined nonaqueous electrolyte from an opening of the outer can 17, and tightly sealing the outer can 17 with the sealing body 18D.
Referring to FIG. 5, the sealing body 18D includes an inverted-dish-shaped terminal cap 19 and a dish-shaped bottom plate 20 both of which are made of stainless steel. The terminal cap 19 has a convex portion 21 protruding outwardly from the battery, and a flat flange 22 serving as the base of the convex portion 21. At the corner edge of the convex portion 21, a plurality of gas vent holes 21a are formed. The bottom plate 20 has a concave portion 23 protruding inward of the battery, and a flat flange 24 serving as the base of the concave portion 23. At the corner edge of the concave portion 23, a gas vent hole 23a is formed.
Accommodated inside the terminal cap 19 and bottom plate 20 is an aluminum safety valve 25 whose shape changes when the battery's internal gas pressure increases to reach a predetermined level. The safety valve 25 has a concave portion 25a and a flange 25b, and made of aluminum foil that is 0.2 mm thick with a 0.005-mm concavo-convex surface, for example. The bottommost part of the concave portion 25a is placed so as to be in contact with the upper surface of the concave portion 23 of the bottom plate 20. The flange 25b is sandwiched between the flange 22 of the terminal cap 19 and the flange 24 of the bottom plate 20. On a part upon the flange 25b of the safety valve 25, a positive temperature coefficient (PTC) thermistor element 26 is provided. The terminal cap 19 and bottom plate 20 are liquid-tightly fixed to each other. Specifically, the flange 24 of the bottom plate 20 is fixed to the terminal cap 19 side with a polypropylene (PP) insulating gasket 27 for a sealing body therebetween, for example.
In the nonaqueous electrolyte secondary battery 10, when an excess current flows in the battery to an extent that causes abnormal heat, the resistance of the PTC thermistor element 26 in the sealing body 18D having a current interrupt function increases, thereby suppressing the excess current. Furthermore, the shape of the concave portion 25a of the safety valve 25 changes when the battery's internal gas pressure increases to reach a predetermined level in order to interrupt contact between the safety valve 25 and the concave portion 23 of the bottom plate 20, thereby interrupting an excess or short-circuit current. It is therefore possible to provide a sealed nonaqueous electrolyte secondary battery that is highly safe.
JP-2000-90892-A discloses a secondary battery for large current discharge including a sealing body whose electrical resistance is kept low. Referring to FIG. 6, which is an enlarged sectional view of this sealing body 40, the body includes from the inside of the battery a dish-shaped bottom plate 41 made of aluminum, a thin-plate safety valve 42 made of aluminum, and a positive electrode terminal cap 43 made of nickel-plated iron that are placed on top of each other. Provided between the bottom plate 41 and safety valve 42 is a ring-like valve retainer 44 made of butyl rubber for tight sealing. The terminal cap 43 and bottom plate 41 are fixed to each other on the periphery, and moreover, the bottom plate, safety valve, and terminal cap are spot-welded at four points 47 with a diameter of 3 mm on a flange 46 on the inside of a fixing member 45 provided to the periphery, thereby completing the united sealing body 40.
Against the background of intensifying calls for environmental protection, regulations on emissions of carbon dioxide and similar gases have been tightened. In the automobile world, development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) is being vigorously pursued in addition to vehicles using gasoline, diesel oil, natural gas and other fossil fuels. Furthermore, the soaring rise in the price of fossil fuels over recent years has given a boost to the development of EVs and HEVs. In addition, sealed batteries represented by lithium-ion secondary batteries have been developed for use in machine tools.
Batteries for EVs, HEVs, and machine tools are required not only to be environmentally friendly, but also to have high-level basic performance as automobiles or tools, that is, large power supply capacity. However, the PTC thermistor element serving as a safety device in nonaqueous electrolyte secondary batteries as disclosed in JP-2001-15155-A restricts the amount of current to be supplied. Therefore, secondary batteries without requiring such a thermistor element have been developed to achieve large power supply.
FIG. 7 is a sectional view illustrating a sealing body 18E having a current interrupt function without requiring a PTC thermistor element. Since this sealing body 18E has the same structure as that of the sealing body 18D shown in FIG. 5 except for the presence of a PTC thermistor element 26, the like numerals indicate like elements in FIGS. 5 and 7 and the description thereof will be omitted here.
If the sealing body 18E having a current interrupt function without requiring a PTC thermistor element is used in the batteries for EVs, HEVs, and machine tools, the batteries can supply large power thanks to their low internal resistance. However, the temperature of the batteries may reach 80 degrees Celsius or more, whereby repeated use may cause heat effects on the PP insulating gasket 27 provided between the terminal cap 19 and bottom plate 20. If the elasticity of the gasket 27 decreases, the contact pressure between the terminal cap 19 and safety valve 25 decreases, thereby increasing or fluctuating the batteries' internal resistance.
As regards the secondary battery for large current discharge disclosed in JP-2000-90892-A, since the positive electrode terminal cap, safety valve, and bottom plate are spot-welded, electrical resistance between the terminal cap and bottom plate is kept low and relatively constant without increasing the area of the battery's output terminal. Accordingly, the battery has an advantage in that its output current can be increased without increasing its weight. However, the sealing body used here has a problem in that a current keeps flowing even if something abnormal happens in the battery, for example, the shape of the safety valve changes or the battery opens with an increased internal gas pressure, since the positive electrode terminal cap, safety valve, and bottom plate are spot-welded and there is no current interrupt means provided between the terminal cap and bottom plate.
Typically, while the positive electrode terminal cap is made of a hard iron-based material, such as nickel-plated iron or stainless steel, the safety valve is made of a thin aluminum-based material with a need for flexibility. The bottom plate is made of aluminum to avoid dissimilar metal contact, since a positive electrode current collecting body in a lithium-ion secondary battery is typically made of aluminum.
To enhance manufacturing efficiency, spot-welding with high-energy rays such as laser or electron beams are widely used these days. Since aluminum has extremely high heat conductivity compared with iron-based materials such as nickel-plated iron and stainless steel, welding with high-energy rays from the terminal cap side, for example, can melt the surface of the terminal cap desirably, while failing to melt the contact portions of the terminal cap and safety valve with an insufficient temperature increase in these portions. Accordingly, it is difficult to strongly weld the terminal cap and safety valve.